The present invention relates to an air inlet for the nacelle of a commercial aircraft jet engine, the said air inlet being coaxial with the said engine and exhibiting the form of an annular lip with a rounded edge, constituting the leading edge of the said nacelle. More particularly, the air inlet of the present invention is of the chamfered typexe2x80x94referred to as xe2x80x9cscarfxe2x80x9d in technical aeronautical languagexe2x80x94in which the leading edge is inclined with respect to the axis of the said engine, the lower part of the said lip projecting forward with respect to the upper part of the latter. A chamfered air inlet such as this, with a forward lower edge, is well known and is for example described in patent U.S. Pat. No. 3,946,830 and in the article xe2x80x9cAerodynamic Performance of Scarf Inletsxe2x80x9d by John M. Abbott, NASA Technical Memorandum 79055, presented at the 17th Aerospace Sciences Meeting at New Orleans, La., from Jan. 15 to 17, 1979.
By comparison with straight air inletsxe2x80x94that is to say the plane of whose leading edge is at least approximately orthogonal to the axis of the enginexe2x80x94chamfered air inlets, with a forward lower edge, exhibit numerous advantages, in particular as regards noise reduction, resistance to the ingestion of foreign bodies and the angle of attack of the lower edge. However, these chamfered air inlets also exhibit certain drawbacks, including very high overspeeds of the airstream feeding the engine, at the internal upper part of the lip under aircraft takeoff conditions, with or without side wind, these overspeeds possibly leading, in the event of side wind and even during a ground run, to the surging of the engine and, possibly, to damage to the latter.
The principal object of the present invention is to remedy these drawbacks and to improve chamfered air inlets, with forward lower edge, so as to avoid these excessive overspeeds.
To this end, according to the invention, the air inlet for the nacelle of a commercial aircraft jet engine, the said air inlet having its axis coincident with that of the said engine and exhibiting the form of an annular lip which comprises:
an external annular aerodynamic surface linking up with the external surface of the said nacelle;
an internal annular aerodynamic surface linking up with the internal surface of the said nacelle and channeling the air to the said engine, the said internal annular aerodynamic surface comprising an annular throat and forming at least in part a diffuser for the said engine; and
a plane annular leading edge formed at the link-up of the said external and internal annular aerodynamic surfaces,
so that any section of the said annular lip through a diametral plane comprises an external profile, an internal profile, a throat and a leading edge corresponding respectively to the traces, in the said diametral plane, of the said external and internal annular aerodynamic surfaces, of the annular throat and of the said annular leading edge,
the said air inlet being symmetric with respect to its vertical diametral plane and being of the chamfered type with the lower part of the said lip forward with respect to the upper part of the latter and the said annular leading edge inclined with respect to the said axis, the inclination of the said annular leading edge being defined, in the said vertical diametral plane, by the scarf angle Sc between the trace of the plane of the said leading edge and the direction perpendicular to the said axis,
is noteworthy in that:
with respect to the two horizontal sections, which are identical and symmetrical, of the said lip, the internal profile of the upper sections of the said lip comprises an internal bulge in the direction of the said axis, the said internal bulge extending on either side of the throat of the said horizontal sections;
the larger the scarf angle Sc, the bigger the said internal bulge; and
the said internal bulge is a maximum at the upper vertical section of the said lip and decreases, progressively and symmetrically, toward the said horizontal sections, where it disappears.
Thus, by virtue of the present invention, the said internal annular aerodynamic surface of the air inlet is thickened, at its upper part, thereby allowing the incident airstream feeding the engine to pass smoothly around the upper part of the leading edge and consequently lowering the corresponding excessive overspeeds of this incident airstream.
It will be pointed out that each section of the said annular lip through a diametral plane making an angle xcex8 with respect to the vertical diametral plane may be defined by:
the rate of contraction CR(xcex8) which is equal to the square of the ratio of the distance of the leading edge from the axis of the engine and of the distance of the throat from the said axis of the engine;
the distance a(xcex8), parallel to the said axis, between the throat and the leading edge;
the distance b(xcex8), perpendicular to the said axis, between the throat and the leading edge; and
the ratio r(xcex8)=a(xcex8)/b(xcex8).
In the present invention, the rate of contraction CR(xcex8) and the ratio r(xcex8) (with xcex8=90xc2x0) of the horizontal sections of the said lip are considered to have base values equal to CRb and to rb respectively.
In this case, in an advantageous embodiment, the air inlet in accordance with the present invention may exhibit the following features:
the rate of contraction CR(0) of the upper vertical section is equal to:
CR(0)=CRb+a1xc3x97Sc,
in which expression a1 is a constant;
the ratio r(0) of the upper vertical section is equal to:
r(0)=rb+a2xc3x97Sc,
in which expression a2 is a constant,
the rate of contraction CR(xcex8) of the upper section of the lip through the said diametral plane making the angle xcex8 with the vertical plane satisfies the equation:
CR(xcex8)=CRb+[CR(0)xe2x88x92CRb]xc3x97cos xcex8
and
the ratio r(xcex8) of the said upper section of the lip through the said diametral plane making the angle xcex8 with the vertical plane satisfies the equation:
r(xcex8)=rb+[r(0)xe2x88x92rb]xc3x97cos xcex8.
When the scarf angle Sc is expressed in degrees, the constant a1 may have a value lying between 0.002 and 0.008 and the constant a2 between xe2x88x920.03 and 0.
In order to make the leading edge of the upper part of the air inlet in accordance with the present invention yet more rounded and thus improve the low-speed flow of the incident air, it is advantageous that:
with respect to the two horizontal sections of the said lip, the external profile of the upper sections of the said lip comprise an external bulge commencing behind the leading edge;
the larger the scarf angle Sc, the bigger the said external bulge; and
the said external bulge be a maximum at the upper vertical section of the lip and decrease, progressively and symmetrically, toward the said horizontal sections, where it disappears.
If, for each upper section of the said annular lip through a diametral plane, we denote by c(xcex8) the distance, perpendicular to the said axis, between the leading edge and the point of the said external profile, whose distance d(xcex8) from the said leading edge, parallel to the said axis, is equal to a tenth of the distance of the leading edge from the axis of the engine, the ratio xcfx81(xcex8)=c(xcex8)/d(xcex8), with xcex8=90xc2x0, corresponding to the horizontal sections presenting the base value xcfx81b, such an external bulge may be characterized by the following features:
the ratio xcfx81(0) of the upper vertical section is equal to
xcfx81(0)=xcfx81b+a3xc3x97Sc,
in which expression a3 is a constant; and
the ratio xcfx81(xcex8) of the upper section of the lip through the said diametral plane making the angle xcex8 with the vertical plane satisfies the equation:
xcfx81(xcex8)=xcfx81b+[xcfx81(0)xe2x88x92xcfx81b]xc3x97cos xcex8.
The scarf angle Sc being expressed in degrees, the constant a3 may have a value lying between 0 and 0.04.
Preferably, in the air inlet in accordance with the present invention:
the internal profile of the lower sections of the said lip may exhibit a greater rate of contraction than the base rate of contraction CRb; and
this greater rate of contraction is a maximum at the lower vertical section of the said lip and decreases, progressively and symmetrically, toward said horizontal sections where it takes the value CRb.
Moreover, to yet further improve the conditions of inlet of the airstream into the said air inlet in accordance with the present invention, it is advantageous that:
the internal profile of the lower sections of the said lip comprise, at the rear of the leading edge, a thinning imposing a rearward retreated position for the throat;
the larger the scarf angle Sc, the bigger the said thinning; and
the said thinning be a maximum at the lower vertical section of the said lip and decrease, progressively and symmetrically, toward the said horizontal sections, where it disappears.